In general, the plastic encapsulation of ICs to form packaged ICs with electrical leads is as follows: Typically, ICs in die form are attached to mounting areas called islands, or die attach pads, on strips termed lead frames in the art. In this specification the die attach pad terminology will be used. The lead frames are typically made of a thin, flat, metal sheet chosen for a number of characteristics including electrical conductivity. Lead frames typically have multiple individual die attach pads, each for supporting an individual IC during a molding operation wherein the individual dies are encapsulated in plastic material, leaving electrical leads protruding from the plastic encapsulation.
In many cases, densely packaged ICs are manufactured to maximize connectivity by utilizing all four sides of the chip. Around the perimeter of each die attach pad a typical lead frame has a pattern of individual conductive leads extending toward, but not contacting, the die attach pad. The die attach pads and individual leads are formed by selective removal of material in the lead frame, such as by stamping. The number of the leads at a frame with a single die attach pad depends directly on the configuration of the particular IC die to be mounted, that is, the number and location of electrical terminations to the die.
A typical IC may well over one hundred external terminations, and each frame will have a corresponding number of individual leads. The width of each lead and the separation between adjacent leads is dependent, among other things, on the package size of the finished IC. The thickness of each lead is the thickness of the lead frame and is predicated partly on the current carrying capacity required.
A plastic package with external leads for connecting to, for example, a printed circuit board, is typically formed by an encapsulation process. Mating molds are placed on each side of the lead frame and liquid-phase polymer is injected to encapsulate IC dies attached to the die attach pads in each frame. The lead frame is designed to dam the flow of liquid-phase polymer as it moves to the outer edges of each individual mold, stopping at the points where each mold contacts surfaces of the lead frame. To stop the flow of liquid-phase polymer between leads the lead frame has a pattern of dam bars between individual leads, so a contiguous band of material is formed around the periphery of the island. This contiguous band prevents the polymer from flooding the entire leadframe, and also allows the lead frame to be one contiguous piece of material until subsequent trimming operations are performed.
After the polymer material solidifies and the molds are removed, a following operation in the manufacturing process removes the excess plastic in the region around the mold outline and the dam bars. This is termed de-junking in the art. A de-damming process then removes the dam bar between each lead, providing electronic integrity for each lead. De-damming is a process of removing all or part of each dam bar by use of a punch with a pattern of teeth conforming to the pattern of the dam bars in the lead frame. Typically, the de-damnming and de-junking can be done in a single step.
In following processing each lead exposed from the edge of the plastic package is further treated such as by plating, and the individual packages are trimmed from the lead frame strip. Finally, the leads are formed, such as for Surface Mount Technology (SMT) applications.
In state-of-the-art manufacturing, automated machines are used to perform the encapsulation process. Automated machines are marketed by a number of manufacturers, including several Japanese manufacturers, and include molds made to close over one or more lead frames, as described above, whereinafter an encapsulation material is injected and caused to solidify. The encapsulation material is typically a liquid-phase polymer material.
There has long been, as is well known in the art, a decided trend toward higher and higher device density on IC chips, and a parallel tendency toward faster and faster operation. Many more devices (transistors) on each chip together with higher speed has quite naturally led to increased power usage, accompanied by a need for heat dissipation. High-end microprocessors, such as those used for computer CPUs operate at power levels in the range of from 25 to 100 watts. At such high power levels there is a critical need to dissipate heat from encapsulated chips at high rates.
As a consequence of the need for heat dissipation, manufacturers, and particularly those who do chip packaging, have developed heat spreaders and heat sinks to be encapsulated within IC packages, and in contact with either the ICs themselves or, more commonly, the die attach pads, to aid in spreading and sinking the heat from operating ICs. The idea in these methods is to increase the thermal mass of the structure, which limits the temperature rise given a particular power usage.
In earlier stages of encapsulation technology molds were typically designed to minimize the amount of material that must be injected. To minimize the encapsulation material, typical dimensions from the inside surface of a formation cavity of an upper half of a mold to the top of a die attached to a die attach pad, and from the inside surface of a formation cavity of a lower half of a mold to the underside of a die attach pad during injection of the liquid-phase polymer while the halves of the mold are closed, are relatively small. A typical dimension for these planned clearances was about 0.010 inches, which is about a quarter of a millimeter.
As device density has risen along with operating speed, providing ICs operating at high power, as described above, molds have been made somewhat larger to accommodate heat spreaders and heat sinks (which may be the same device in many cases). In use, such spreaders and sinks are typically placed in a lower mold portion such that placing a lead frame in position, the lead frame having ICs attached to the die attach pad (island) portions of the lead frame, brings the die attach pad in contact with the spreader/sink. Then an upper mold portion is closed on the lower mold portion such that the ICs on die attach pads are enclosed in individual molds, and semi-liquid polymer material is injected. The intent is that after injection and solidification of the polymer material, the heat sink/spreader will still be in contact with the die attach pad, so heat developed within the IC die in operation can be efficiently transferred to the heat sink/spreader.
Unfortunately, keeping a sink/spreader in position is no simple task. Throughput demands that injection must take place as quickly as practical, and, although effort is expended in mold design, there are always flow pattern that exert forces on the IC dies, the die attach pads, and on any heat sink/spreader that may be within a closed mold when injection takes place. These forces all too often move the components within the mold, and separate a spreader from its ideal position relative to a die attach pad and a mounted die either before solidification or as the polymer material solidifies. The result is often a defective IC package that will not operate at full spec below a maximum expected temperature for the operating IC die. In some cases dies fail due to high temperature, and in other cases performance is adversely affected.
Heat spreaders and heat sinks of many sorts have been developed and are in use at the time of the present patent application, but at the time of this application placement and positional integrity of heat sinks and spreaders is still a problem. What is needed is a heat sink/spreader that stays reliably in position, providing effective heat sinking after the processed packages are in service and operating at maximum power.